The present invention relates to the manufacture of grades of steel which do not undergo phase transformation when being heated for hot working and, in particular, to a method of conditioning cast bodies of such steels to recrystallize upon heating.
In the manufacture of steel as presently practiced commercially, the molten steel is first formed into a body by casting, for instance, as an ingot in an ingot mold, or as a slab or billet of uniform cross-section by continuous casting or by pressure casting. The cast body is then processed to an intermediate product, usually a slab, plate, band or billet, by hot working.
Solidification of the molten steel in an ingot mold, continuous casting mold or pressure casting mold begins next to the mold wall by the formation of a thin surface layer of small equiaxed grains, some of which form the nucleii for large columnar crystals or grains (dendrites), that grow in a direction opposite to that of the heat dissipation. Due to segregation, various impurities contained in the melt end up in parts of the solidified cast body which were the last to solidify, that is, long the grain boundaries. The grain boundaries in alloys are zones of weakness, sites for oxidation or other adverse effects upon heating in preparation for hot working and cracking resulting therefrom during hot working.
Most steel grades undergo phase transformations in the course of cooling down after casting and of being reheated for hot working. Each phase transformation has associated with it a recrystallization which usually results in the formation of a finer, stronger grain structure. The recrystallizations that occur during temperature changes in most steel grades greatly reduce the propensities for oxidation and other attacks along grain boundaries upon heating and for cracking during working that would exist if the initial grain structure of the cast body were not significantly reformed by phase transformation and resulting recrystallization.
Certain metals and alloys do not, however, exhibit such phase transformations upon undergoing temperature changes. An important class of steels having the characteristic of phase stability through the range of temperatures applicable to hot working is the AISI 300 series, the austenitic stainless steels. The austenitic stainless steels solidify predominently as ferrite, all or most of which (depending on the composition) undergoes a phase transformation into austenitie immediately below the solidification temperature range. The essentially austenitic structure inherits the coarse columnar grain configuration and remain stable (without any phase transformation or recrystallization) as the steel is further cooled down after casting or heated up for hot working. The lack of recrystallization of the austenitic stainless steels during cooling subsequent to solidification and heating for hot working means that the coarse grain structure of the initially cast material must undergo heating and initial working. The coarse grain structure is prone to oxidation and other attacks along the grain boundaries near the surfaces as the cast body is heated for hot working and is subject to cracking during working. Such cracks can show up as edge cracks and/or silvers in the hot-worked intermediate product.
The molybdenum-containing grades, such as AISI 316 and 317 and modifications thereof, appear to be particularly subject to damage in the initial hot working phase of the processing of the cast body. It is believed that heating of those grades prior to hot working results in preferential oxidation of molybdenum along the grain boundaries near the surface and, thus, causes a further weakening of those boundaries. In all grades of austenitic stainless steel the susceptibility to surface and edge defects due to intergranular cracking also appears to be increased by the presence of metallic and non-metallic contaminants along the grain boundaries.
Following the initial hot working of the cast product to produce the intermediate product, e.g., a slab, plate, band or billet, the intermediate product is ordinarily conditioned to remove scale and surface defects. That operation is costly and time-consuming and involves the loss of material; the greater the extent of defects arising from cracking and slivers, the greater the cost in time and loss of material in the conditioning operation. In some cases the conditioning step can disrupt production schedules and delay deliveries. From time to time, there may be so much material removed in conditioning that the desired end product cannot be made from the intermediate product; for example, a slab destined to be made into a certain size plate may lose so much weight in conditioning that it will have to be down sized to a smaller sized plate at a substantial loss.
One approach to improving the initial hot working properties of cast austenitic steel has been based on cold working in the form of shot blasting the surfaces of the cast body prior to the initial hot working. Shot blasting affects only a thin layer at the surface without any noticeable change in cross-sectional area, and penetration is not significant except with shot of sizes so large that the shot blasting equipment cannot endure it. The treatment is also very costly.